1. Field of the Invention
This invention relates broadly to radio communication systems. More particularly, this invention relates to Personal Communication Service (PCS) cellular communication systems, such as CDMA systems, that employ a beacon signal (e.g., a pseudo-noise digital sequence in CDMA) to communicate from base stations to mobile units within the system.
2. State of the Art
Code Division Multiple Access (CDMA) cellular systems employ a digital wireless transmission method in which pilot signals are transmitted from base stations to mobile units. The pilot signal is an unmodulated, direct sequence spread spectrum signal transmitted continuously by a base station. The pilot signal allows a mobile station to acquire the timing of the forward channel (from the base station to the mobile unit), provides a phase reference for coherent demodulation, and provides a means for making signal strength comparisons between base stations for determining when to handoff. The pilot signal is a Walsh code “0” spread by a pair of modified maximal length pseudo-noise sequences. The relative phase of the pseudo-random sequence pair corresponds to different forward channels within a common frequency band (which is centered around 800 MHz or 1900 MHz for current systems). This pseudo-random sequence pair is typically referred to as a short PN sequence or code. The short PN sequence has a period of exactly 215 (or 32,768) chips and the relative phase of the short PN sequences are offset by an integer multiple of 64 chips (which is 52.083 μsec). Therefore, there are exactly 32,768/64 or 512 phase offsets of 64 chips each. One of the possible 512 phase offsets is assigned to each base station. This phase offset, typically referred to a PN offset, allows the mobile unit to identify that base station by its offset.
The synchronization of the base stations of the CDMA system is achieved by the use of Global Positioning System (GPS) receivers at each base station location. Aided by appropriate stable clock generators, these GPS receivers supply accurate timing information to the base station.
Pilot pollution occurs within the coverage area of the CDMA cellular system when numerous pilot signals are received with relatively equal signal strength. Such pilot pollution is detrimental because it may cause dropped calls and decreased capacity. Thus, power optimization is advantageous as it reduces pollution from different offset pilots. Such optimization is typically accomplished by drive-testing the intended coverage area of the CDMA cellular system with a pilot scanner that detects the received pilot signals and measures/records the signal strengths of the detected piloted signals at various locations within the intended coverage area of the CDMA cellular system. The pilot scanner requires access to an accurate clock source. Typically, the Global Positioning System (GPS) is used as the clock source. As such, these devices typically have GPS receivers. Whenever the GPS signals are available, that is in most outdoor settings, the GPS receiver provides an accurate and convenient way of synchronization. However, it is not practical for settings where there is no reliable GPS coverage, such as indoor settings, some urban locations and wherever access to GPS is obstructed.
Another possible approach, which is suitable for indoor analysis where access to GPS is obstructed, relies on the information contained in the CDMA signal itself to derive the accurate frequency and timing information. In this approach, the scanner demodulates the synchronization channel and recovers the PN offset from the body of the L3 message within the synchronization channel. The time-of-arrival of the pilot that corresponds to the recovered PN offset is used as a reference for the timing of the other detected pilot signals. Such timing is then used to determine the PN indices for the other detected pilots. The pilot that is selected for synchronization channel processing plays an important role in the correct detection of the other pilot signals. If this selected pilot originates from a base station that is very far from the pilot scanner, the timings that are referenced to the selected pilot will typically have large errors that lead to further errors in the determination of PN indices based thereon. Thus, in certain circumstances, this approach fails to accurately identify the PN indices of the detected pilots.
Thus, there remains a need to provide an improved apparatus and methodology for evaluating CDMA signal propagation and coverage, including pilot number offset measurements, in indoor and similar settings with no or limited access to GPS signals.